An Orthogonal Frequency Division Multiplexing (OFDM) method is a multi-carrier transmission method which divides an entire usable frequency band into a predetermined number of narrow bands, modulates the sub-carriers of the narrow bands in parallel, and transmits the modulated sub-carriers. To each sub-carrier, low-rate data, which has a small amount of data, are allocated. The applied modulation methods are diverse from simple Quadrature Phase Shift Keying (QPSK) to 256-Quadrature Amplitude Modulation (QAM) according to the change of data capacity or definite transmission requests.
Channel signals have orthogonality to approach another channels without causing interference. Since no other sub-carrier can affect the center frequency of each channel, the frequency utility efficiency is high. Since each sub-carrier is processed into narrow band signals, such as 1 Khz, the transmission rate is low. So, although time delay is caused, such as 500 nanoseconds, while the channel signals are multi-reflected and transmitted, the interference between the OFDM symbols can be removed.
In other words, the sub-carriers used in the OFDM method have orthogonality, the frequency utility efficiency is increased and multi-path channel problem can be overcome only with a simple frequency domain equalizer having one tab. Since the OFDM method can be implemented at a high speed using Fast Fourier Transform (FFT), the OFDM method is used as a transmission method for high-speed digital communication systems recently.
For example, the OFDM method is used in wireless communication systems, such as Digital Audio Broadcasting (DAB), Digital Video Broadcasting (DVB), Institute of Electrical and Electronics Engineers (IEEE) 802.11a, and High Performance Radio Local Area Network 2 (HIPERLAN/2). Also, Discrete MultiTone (DMT), which is similar to the OFDM, is used in wired communication systems, such as x Digital Subscriber Line (xDSL).
Meanwhile, differently from a communication system adopting a broadcasting method or a point-to-point method, when a plurality of mobile stations transmit data using the OFDM method, multiple-access methods are needed, for example, OFDM-Time Division Multiple Access (TDMA), OFDM-Frequency Division Multiple Access (FDMA) (OFDMA), and OFDM-Code Division Multiple Access (CDMA).
In the OFDMA, each mobile station can use predetermined sub-carriers among the entire sub-carriers all the time. The sub-carriers can be allocated variably according to the requests from the mobile stations. To put it differently, in the OFDMA, resources can be distributed efficiently by allocating sub-carriers differently according to a data transmission rate requested by each mobile station. The OFDMA provides a high transmission efficiency, because it does not require any preamble that is requested in an OFDM-TDMA system.
Particularly, the OFDMA method is suitable when a large number of sub-carriers are in use, that is, when the amplitude of an FFT unit is large. So, it can be applied efficiently to a wireless communication system having a cell of wide area that is a cell whose delay spread is relatively large.
Meanwhile, frequency hopping OFDMA (FH-OFDMA) is used to increase frequency diversity effect and obtain interference average effect by overcoming sub-carriers in deep fading or the sub-carrier interference by another mobile station. More details on this are described in a book by Richard van Nee and Ramjee Prasad, entitled “OFDM Wireless Multimedia Communications,” Artech House, 2000.
FIG. 1A is a diagram showing a frequency hopping pattern of a cluster in accordance with a conventional OFDMA method. Referring to FIG. 1A, different frequency bands a, b and c are allocated according to a data transmission rate requested by a mobile station. The allocated frequency band is changed by performing frequency hopping based on time. The vertical axis 11 of each rectangular cell of FIG. 1A is a set, of consecutive sub-carriers in the frequency domain, that is, a frequency band which is the number of sub-carriers within a rectangular cell * sub-carrier frequency interval (cluster), and the horizontal axis 10 of each rectangular cell indicates a symbol period.
According to the conventional FH-OFDMA method, a predetermined number of neighboring sub-carriers are grouped among the entire sub-carriers to form a cluster and allocated to mobile stations on a cluster basis. The cluster performs frequency hopping based on a time slot so that the cluster could not fall into frequency null continuously.
More detailed description is presented in an article by J. Chuang and N. R. Sollenberger, entitled “Beyond 3G: Wideband Wireless Data Access Based on OFDM and Dynamic Packet Assignment,” IEEE Communication Magazine, pp. 78-87, July 2000.
FIG. 1B is a diagram showing clusters falling into frequency null during frequency hopping in the conventional OFDMA method. Referring to FIG. 1B, the clusters 40 and 41 perform frequency hopping randomly based on a time slot. Each cluster is a set of consecutive sub-carriers. When a cluster 40 falls into frequency null of a channel as shown in time slot 3, burst errors occur. To overcome the burst errors, interleaving or encoding is performed.
However, the conventional method using clusters has a problem that the mobile station consumes a great deal of power because it performs FFT with respect to the entire sub-carriers, even if it has a cluster assigned to itself. Also, the conventional method cannot overcome the burst errors in case that the data of a packet are not long enough to perform interleaving, such as a control signal.